GSTM1 and myeloproliferative neoplasm: Even with a small patient sample, we were able to see that our population of PV patients had characteristics and outcomes that were similar to those of patients treated in centers in the United States and Europe.4,11,16 Although application of the WHO criteria allows PV diagnoses to be made only in the presence of elevated Hb levels associated with the existence of other characteristics compatible with chronic myeloproliferative disorders, some patients may require red cell mass measurement in order to correctly establish the diagnosis.